Breaking down weed surface tension

However, not all weeds have the same waxy or hair surface, McCloskey told the 11th annual Desert Vegetable Crop Workshop held recently in Yuma, Ariz., sponsored by the UA, University of California and Western Farm Press.

While there is variability in waxy surfaces of broadleaf weeds, most grasses have very waxy surfaces, particularly summer grasses like Johnsongrass.

Non-ionic surfactants are the most widely used in agriculture. However, McCloskey said to avoid organo-silicone surfactants, which can actually impede the efficacy of Roundup, especially in controlling grasses. One reason for that is that it spreads droplets too thin and under desert arid conditions the herbicide evaporates too quickly.

However, nutsedge does not have as much wax coating on its leaves, yet it is one of the most difficult weeds to control with glyphosate, according to McCloskey, who did his research based on the increasing use of that herbicide and other contacts.

The proper micro size helps disperse the surfactant and herbicide tank mix. McCloskey recommends 200 to 400 micron size. "The smaller droplet size, the better," McCloskey said. "However there is always the need to balance the need for good coverage against the chance of drift."

Water needs

Most herbicides perform best with water gallonages of 10 to 43 gallons per acre. The exception is glyphosate, which is most effective in the 3 to 10 gallon range, noted McCloskey.

Adding ammonium sulfate to the tank before pouring in glyphosate also can enhance the herbicide’s efficacy, but the 17 pounds per 100 gallons recommended on the label is too much, according to McCloskey.

UA researchers analyzed a wide array of well and surface water sources in Arizona for calcium, magnesium, sodium and potassium, the most antagonistic elements in water to glyphosate.

It only took about 8.5 pounds of ammonium sulfate to buffer the lowest quality water, which had 1,000 ppm salts, according to McCloskey.

"You can analyze your water and use the chart developed by the university to add the correct amount of ammonium sulfate. If you do not know your water quality, adding 8.5 pounds per 100 gallons of water should be adequate to do the job," said McCloskey.

"Ammonium sulfate is not that expensive, but it may save a little if you add only what you need," McCloskey said.